Effects of Deep Convection on Upper Tropospheric Outflow Ice Supersaturation and Cirrus Cloud Formation

A barrier in constraining the Earth's radiative forcing budget stems from the large uncertainties associated with cloud formation and dynamics. Recent work* has shown that small scale dynamics play a significant role in controlling the relative humidity of the upper troposphere and, in turn, the microphysics of cirrus clouds. While there has been significant discussion of the long-term transport effects of ground level trace gases and aerosols, only recently have datasets become available which examine the effects of fast convective transport on the relatively pristine upper troposphere. During the NSF Deep Convective Clouds and Chemistry (DC3) Experiment in May-June 2012, multiple aircraft, each with a large suite of chemical, aerosol and, cloud physics payloads, were utilized to characterize both the inflow and outflow of deep convective storms over the continental US. We have used data from 10 storms during DC3 as case studies to illustrate the influence of trace gases and aerosols, transported by deep convection to the upper troposphere, on ice supersaturation regions and cirrus cloud formation. Ice supersaturation regions (ISSR), defined as regions with relative humidity greater than 100% at temperatures below -40°C, in the outflow region of each storm are identified using humidity data from the NSF/NCAR VCSEL hygrometer on the NSF G-V. The ISSR intensity of the outflow of a storm is defined as the aggregate mean of the maximum relative humidity encountered in each individual ISSR in this region, a quantity that is observed to increase with ISSR length scales. Coordinated sampling of the inflow region of each storm, determined from NEXRAD radar measurements and flight tracks combined with notes from the flight summaries, by the NASA DC-8 provide a characterization of the chemical and particulate composition at the base of the storm. Mineral and nitrate particulate in the storm inflow are observed to have strong positive correlations with the ISSR intensity in the outflow regions, suggesting a link between these species and inhibited ice nucleation. Oil particulate and gas phase alkyl nitrates are observed to have strong negative correlations with ISSR intensity, suggesting a link between these species and enhanced ice nucleation. This is in contrast to our current understanding of the nucleation properties of these chemical and aerosol species, and current work is focused on explaining this apparent discrepancy. Furthermore, we discuss the potential impact of these findings on cirrus cloud formation during convective outflow. *Diao et al., Geophys. Res. Lett. (2013)